1. Field of the Invention
The present invention relates to a superconducting thin film of compound oxide and a process for preparing the same. More particularly, it relates to an improved superconducting thin film of Bi-containing compound oxide deposited on a substrate and a process for preparing the same.
2. Description of the Related Art
When oxide superconductors are utilized in electronics devices or the like, it is indispensable to prepare their thin films. Thin films of the compound oxides deposited on single crystal substrates of oxides such as SrTiO.sub.3, MgO or the like exhibits relatively higher superconducting property because of the following reasons:
Firstly, bad influence of diffusion or migration of elements of which the substrate is made is relatively lower, although such diffusion or migration of substrate elements is inevitable when the compound oxides are deposited on substrates.
Secondly, it is rather easy to prepare well-oriented or well-ordered thin films of polycrystals or single crystals of superconducting compound oxides when these compound oxides are deposited on predetermined planes of such single crystal substrates of SrTiO.sub.3, MgO or the like. In fact, so-called epitaxial growth is rather easily realized when the thin films of compound oxides are deposited on such single crystal substrates of SrTiO.sub.3, MgO or the like, so that the superconducting thin films deposited on these substrates become single crystals or polycrystals which have very well ordered crystalline structure and show improved superconducting property. Still more, anisotropy of the superconducting property which is inherent in these compound oxide superconductors can be controlled.
Film formation of these oxide superconductors is effected usually by physical vapour deposition technique such as sputtering, ion-plating and chemical vapour deposition technique such as MO--CVD.
However, oxide superconductor thin films prepared by known processes are difficultly applicable to electronics devices because their surfaces are not smooth. Such surface unevenness of superconductor thin film may be caused by influence of surface roughness of the substrates and by mismatch of lattice constants between substrate crystal and superconductor crystal.
Namely, the surface of the single crystal substrate of oxide is not smooth in the atomic level. In fact, even if the surface of the single crystal substrate of oxide is polished completely, its diffraction pattern observed by a reflective high-energy electron diffraction analyzer (RHEED) is a spotty pattern but is not a streaky pattern which reflect surface smoothness. The mismatch of lattice constants between substrate crystal and superconductor crystal is another cause of surface unevenness because a stress in the thin film which can be absorbed at an early stage of film formation is released when the thin film becomes thicker.
U.S. Pat. No. 4,837,609 proposes to insert a layer of W, Mo or Ta between a superconducting compound oxide layer and a silicon single crystal substrate.
Japanese patent laid-open No. 63-239,840 proposes to oxidize a copper substrate to form a CuO layer thereon and then a superconducting compound oxide layer is deposited on the CuO layer.
These prior arts, however, neither describes not mentions surface smoothness of the superconducting thin films.
Therefore, an object of the present invention is to solve the problems of known processes and to provide an improved oxide superconductor thin film having a smooth surface.